1. Technical Field
This invention relates generally to thin film resistors on aluminum nitride substrates and, more particularly, to a method for trimming the thin film resistors.
2. Discussion
Thin film resistors are used extensively in integrated circuits having resistors which require very precise resistance values. By controlling the deposition of a thin film of resistive material on an insulating substrate, the resistivity of the film as well as the resulting resistance can be controlled to within five percent of a target resistance. For circuits requiring even more precise resistances, laser trimming is the established method for providing fine adjustments to the resistance of the thin film resistor. By using a laser beam to etch away a predetermined pattern, or "trim kerf", of the resistive material from the substrate, the available conductive path is reduced and the resistance is, therefore, increased. Active trimming processes in which actual circuit performance is monitored as the result of trim can be used to adjust resistance to within one tenth of one percent. Precision of the trim monitoring equipment normally becomes the limiting factor in resistance accuracy.
When an aluminum nitride substrate is used, however, unpredictable results have been obtained from the laser trimming process. Decreases in resistance after laser trimming have been found to occur, possibly as a consequence of the transformation of the aluminum nitride into highly conductive aluminum metal. An improved method for providing fine adjustments to the thin film resistor formed on an aluminum nitride substrate is, therefore, required.